1. Field of the Invention
The present invention relates to a vacuum cleaner, and more particularly, to a safety device by which a driving motor for generating suction force in a vacuum cleaner cannot be overloaded.
2. Description of the Prior Art
As shown in FIG. 1, a vacuum cleaner is generally constructed to comprise a main body 2, a connecting hose 4 connected to the main body 2, an extension tube 6 connected to the connecting hose 4, and a suction nozzle 8 mounted to a lower end of the extension tube 6.
A driving motor for generating suction force and a filtering device for filtering air containing foreign materials to be introduced are included in the main body 2. The connecting hose 4, which is manufactured in the form of a pipe made of flexible material, causes the main body 2 to be connected to the extension tube 6. The extension tube 6 is generally constructed to be extendable and contractible. Further, a handle gripped by a user is formed at an upper portion of the extension tube.
The suction force generated from the main body 2 is transferred to the suction nozzle 8 via the connecting hose 4 and the extension tube 6. In addition, since the suction nozzle 8 is usually used in a state where it is in contact with a surface to be cleaned, the foreign materials such as dusts on a room floor is introduced into the main body through the suction nozzle 8 together with the air. The air containing the foreign material introduced as such is introduced into the main body through the extension tube 6 and the connecting hose 4, and then is filtered.
FIG. 2 shows an interior structure of the main body 2 of the cleaner in a state where an upper casing of the main body 2 is opened. Referring to FIG. 2, the interior structure of the main body will be explained. As shown in this figure, a dust-collecting chamber 22a into which a dust-collecting bag such as a paper bag is inserted is provided within a lower casing 21 constructing a lower portion of the main body 2. The dust-collecting chamber 22a is divided by a partition 26, while a chamber 22b for accommodating the motor (hereinafter, referred to as a “motor chamber”) is formed at the other side of the partition 26. A motor housing 24 in which the motor for generating the suction force is mounted is installed within the motor chamber 22b. 
A motor cover 25 for opening and closing an upper portion of the motor housing 24 is provided with a connecting tube 25a that communicates with the motor chamber 22b located below the motor cover. Further, a dust-collecting bag (not shown) for allowing only the air, out of the air containing the introduced foreign materials, to be discharged to the outside is installed within the dust-collecting chamber 22a. Furthermore, a cover 28 in which a damper 28a is mounted is installed at an upper portion of the dust-collecting chamber 22a. The damper 28a has a construction that the damper is opened when pressure within the dust-collecting chamber 22a becomes lower than the atmosphere pressure, so that air outside the main body of the cleaner can be introduced into the dust-collecting chamber 22a. The structure of the damper 28a itself is well known, and thus, detailed description thereof will be omitted.
As shown in FIG. 3 in which a connecting relationship between the connecting tube 25a and the partition 26 is illustrated, the connecting tube 25a connected to the motor cover 25 is connected to a vent hole 26a formed in the partition 26, and a pressure switch 27 for sensing pressure within the dust-collecting chamber 22a is connected to an end of the connecting tube 25a. Thus, the pressure switch 27 is formed through the vent hole 26a so that it can sense the pressure within the dust-collecting chamber 22a. 
The suction pressure generated when the motor installed within the motor housing 24 starts to operate is transferred to the suction nozzle 8, and consequently, the foreign materials such as dusts remaining on the room floor are introduced into the main body 2 through the suction nozzle 8. Further, only the air out of the air containing the foreign materials, which has been introduced into the dust-collecting bag inserted into the dust-collecting chamber within the main body, flows from the dust-collecting bag. Thus, the foreign materials such as dusts are gradually collected within the dust-collecting bag.
In addition, the air flowing from the inside to the outside of the dust-collecting bag passes through the interior of the motor housing 24, and is discharged to the outside of the cleaner. The reason why the air passes through the interior of the motor housing 24 is that the air should cool down the motor from which heat is generated during its operation.
If the dusts are collected in a large quantity within the dust-collecting bag or large foreign materials are caught in any one of the connecting nozzle 8, the extension tube 6 and the connecting hose 4 in the process of such an operation, the pressure within the dust-collecting chamber 22a is lowered. That is, the pressure within the dust-collecting chamber 22a is lowered, because the motor installed within the motor housing 24 incessantly performs its own discharge operation whereas a small quantity of the air is introduced into the dust-collecting chamber 22a. 
Then, if the pressure is lowered, the motor installed within the motor housing 24 is substantially overloaded. That is, this is because the motor discharges the air within the dust-collecting chamber while operating in such a state where the air cannot be normally introduced into the dust-collecting chamber 22a due to the foreign materials sucked into the dust-collecting bag inserted into dust-collecting chamber. However, such an overload is not preferable since it exerts a critical influence on the function of the cleaner.
Therefore, in this case, the pressure switch 27 mounted to sense the pressure within dust-collecting chamber 22a through the vent hole 26a in the partition 26 can sense reduction of the pressure within the dust-collecting chamber. Further, based on the pressure sensed by the pressure switch 27, alarm lights or sounds can be produced to inform the user that the cleaner is in an abnormal state. Thus, the user can take measures such exchange of the dust-collecting bag.
On the other hand, in a case where the user cannot perceive such an alarm, the motor installed within the motor housing is continuously overloaded. Thus, the damper 28a mounted on the cover 28 starts to operate. That is, if the pressure within the dust-collecting chamber 22a is greatly lower than the outside pressure, the damper 28a is opened and the air outside the main body of the cleaner is then introduced into the dust-collecting chamber 22a so that the motor is prevented from being overloaded.
However, the vacuum cleaner constructed as such has the following problems.
The aforementioned damper 28a and pressure switch 27 are parts for eventually preventing the motor from being overloaded. According to the prior art mentioned above, however, the pressure switch 27 senses merely the pressure within the dust-collecting chamber 22a, and moreover, it measures the pressure through the connecting tube 25a. Thus, there is likelihood that any error in measurement of the pressure can be produced. Consequently the pressure cannot be accurately measured.
Further, since the air introduced through the damper 28a is introduced into the motor housing 24 via the dust-collecting chamber 22a and the dust-collecting bag, a sufficient quantity of the air is not substantially introduced into the motor housing 24. Thus, there is a disadvantage in that the motor cannot be sufficiently prevented from being overloaded.
Furthermore, the aforementioned conventional constitution is not substantially simple and requires the other components such as tubes. Thus, there is also a problem in that the number of parts is increased and fabricating processes become complicated.